<template>
  <ul class="list">
    <li class="item" v-for="(item,index) of letters" :key="index"
        @click="handleLetterClick"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        :ref="item">
      {{ item }}
    </li>
  </ul>
</template>

<script>
  export default {
    name: 'CityAlphabet',
    props: {
      cities: Object
    },
    computed: {
      letters(){
        const letters = []
        for (let i in this.cities) {
          letters.push(i)
        }
        return letters

      }
    },
    data(){
      return {
        touchStatus: false,
        startY: 0,
        timer: null
      }
    },
    updated(){
      //当页面数据被更新时，同时完成渲染时updated钩子就被执行
      this.startY = this.$refs['A'][0].offsetTop
    },

    methods: {
      handleLetterClick(e){
        this.$emit('change', e.target.innerText)
      },
      handleTouchStart(){
        this.touchStatus = true
      },
      handleTouchMove(e){
        if (this.touchStatus) {
          if (this.timer) {
            clearTimeout(this.timer)
          }
//          函数节流，提高网页性能
          this.timer = setTimeout(() => {
            const touchY = e.touches[0].clientY - 79
            const index = Math.floor((touchY - this.startY) / 20)
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index])
            }
          }, 16)

        }
      },
      handleTouchEnd(){
        this.touchStatus = false
      }
    }
  }
</script>


<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      line-height: .4rem
      text-align: center
      color: $bgColor
</style>
